My invention comprises improvements in a power steering gear of the kind disclosed in my pending application Ser. No. 046,701, filed April 16, 1993, which is a continuation-in-part of my patent application Ser. No. 811,963, filed Dec. 23, 1991, entitled "Electronic Power Assist Control" now abandoned. Both of these patent applications are assigned to the assignee of the present invention.
As in the case of the power steering gear disclosed in my co-pending application Ser. No. 046,701, the steering gear of my present invention comprises a fluid motor for actuating dirigible wheels of a motor vehicle. Steering pressure for the fluid motor is developed by a power steering pump driven by the vehicle engine. A rotary valve assembly is used to control pressure distribution from the pump to the motor to effect a power steering assist in either direction during turning maneuvers. The rotary valve assembly has an inner valve member connected mechanically to the steering shaft and a rotary valve sleeve surrounding the inner valve member. The valve sleeve is connected mechanically to the driven portions of the steering gear mechanism as steering torque is delivered to the dirigible wheels. A torsion bar resiliently resists relative angular displacement of the sleeve and the inner valve member.
The valve assembly comprises registering valve lands on the inner valve member and in the sleeve. The valve lands cooperate to control pressure distribution from the pump to either of two sides of a double-acting piston of a fluid motor. Pressure developed in the pressure passage connecting the pump with the motor is distributed to each of two pressure working chambers defined in part by the double-acting piston. When the pressure in one working chamber increases and the pressure in the other working chamber decreases, the vehicle is adapted to turn in one direction. Upon a reversal in the direction of the pressure differential in the fluid motor, the steering direction reverses.
Effective flow area across the valve lands of the rotary valve mechanism changes as steering torque is applied. Since the flow delivered by the pump is constant regardless of pump speed, the pressure differential developed in either one working chamber or the other is a function of the reciprocal of the square of the effective flow area across the valve lands. This characteristic is described in my co-pending application identified above.
A vehicle speed sensitive steering gear is described in U.S. Pat. No. 4,570,736. The relationship between torque applied to the input shaft and the steering pressure developed in the fluid motor of the '736 patent can be changed by a vehicle speed sensitive valve arrangement in which a parallel flow path from the pump to the valve system is established at high speeds, thereby making the pressure sensitive to changes in torque. As the vehicle speed increases, the flow through the parallel flow circuit is increased thereby providing a reduced power assist at high speeds.
In a steering system such as that shown in prior art U.S. Pat. No. 4,570,736, it is necessary to overcome the effect of lateral acceleration during a turning maneuver at any given speed. Thus, the steering assist during straight ahead driving at a given speed will be less than the steering assist that is available during a turning maneuver at that same speed because the effort that must be applied to the steering shaft must be greater during the turning maneuver in order to overcome the effects of lateral acceleration of the vehicle. This increased force results in an increase in steering pressure. The vehicle operator compensates for the change in steering effort that is required to execute a turning maneuver compared to the steering effort that is required for steering corrections at the same vehicle speed during straight ahead driving.
The steering system described in my co-pending application provides a different steering characteristic than that which is described in prior art U.S. Pat. No. 4,570,736. The resistance to steering effort in the design of my co-pending application is determined by a modulated steering pressure that acts on reaction pistons. This opposes relative movement of the steering valve elements. The steering pressure then will increase linearly during torque increases. A pressure modulator valve is actuated when the steering pressure reaches a calibrated value. The output of the modulator valve then is distributed to the reaction pistons. At any steering torque in excess of the value that triggers the operation of the modulator valve, the differential steering pressure resulting from an incremental increase in steering torque will increase at a greater rate. This change from one pressure/torque relationship to the other is characterized by a so-called breakpoint.
When the vehicle is operated at high speeds and the steering torque increases to a value greater than a calibrated value, the steering pressure applied to the reaction pistons increases at a reduced rate, thereby reducing the torque increase for a given steering pressure increase.